function MDCalc(z) {
	if (pref == "si") {
		h2o = 6.27
	}
	if (pref == "us") {
		h2o = 47
	}
	value = z.fio2 / 100 * (z.atm - h2o) - z.paco2 / .8;
	ans = value - z.pao2;
	Answer(Round(ans, 1));
	if (z.age) {
		if (pref == "si") {
			Text("抱歉，在国际单位制下，肺泡－动脉分压差不能用年龄来估计。")
		} else {
			val = z.age / 4 + 4;
			Answer2(Round(val, 1))
		}
	}
}